WHEN IS A QUADRATE NOT A QUADRATE?

["Matthew Troutman" <m_troutman@hotmail.com>]

I have just received a most interesting paper:

Welman, J. 1995. _Euparkeria_ and the origin of birds.  South African 
Journal of Science. 91: 533-537.

Of the many interesting points inside the paper, Welman says that 
theropods and crocodylomorphs are too specialized to be the ancestors of 
birds (the possession of a derived median eustachian system is one 
feature; last time I checked,  it was considered  homologous to the 
basisphenoidal sinus and basisphenoid recess) and _Euparkeria_ is the 
quintessential avian ancestor based on braincase features.  I don't buy 
it for a second, but it is rather compelling and the paper is a good 
mine of information on avian braincases (with a few errors). 

Anyway, to the title of the post... Welman also makes a most interesting 
interpretation of the 'unidentified skull bone' of the London 
_Archaeopteryx_; it is a broken piece of the left side of basicranium.  
This is of interest because many recent authors (Walker and Elzanowski 
and Wellnhofer) have identified it as a quadrate.  The 'pterygoid 
flange'-like structure is a basipterygoid process, the broken base is a 
alaparasphenoid with a rostral tympanic recess (a very birdlike 
feature), and the dorsal head is the basiparasphenoid (compare to 
braincase floor in Fig. 2A of Elzanowski and Wellnhofer 1996).  Most 
interesting is this interpretation because it reopens the issue of 
whether or not _Archaeopteryx_ has a bipartite quadrate articulation; 
the seventh specimen, contrary to some people, is indeterminate in the 
matter of a single-headed or double-headed quadrate:

"There is no indication of the medial (prootic) head although its 
presence cannot be positively ruled out as the otic process is damaged 
in this area."  Elzanowski and Wellnhofer; 86.  

Elzanowski and Wellhofer say that the _Archaeopteryx_ quadrate is 
single-headed only because of the interpretation of the London 
'unidentified skull bone'. 

So, from my viewpoint, the issue of a single or double head in the 
_Archaeopteryx_ quadrate is up in the air.  

MORE INTERESTING STUFF

I know from first hand experience that it is very hard to impossible to 
tell whether a bird quadrate is double-headed on the skull is viewed in 
lateral view.  Heck, they don't call it the medial quadrate head for 
anything.  Also, the medial head is not easily viewed even in a frontal 
view of the avian skull, since it is occasionally very high on the avian 
quadrate and sometimes hidden by the laterosphenoid and ectemoid. 
Occasionally, the quadrate of ratites appears single-headed and only a 
close examination shows that the quadrate is double-headed.

Another interesting bit of evidence for the presence of a double-headed 
quadrate in _Archaeopteryx_ is the weird flat threshold to the caudal 
tympanic recess on the London paraoccipital process (the structure of 
the threshold to the caudal recess is different from various birdlike 
theropods); Whetstone interpreted it as a quadrate articulation area but 
Walker suggested otherwise since there was evidence for a fairly typical 
archosaurian quadrate articulation rather than the articulation 
suggested by Whetstone.  Anyway, it can still be interpreted as a 
quadrate cotyle.  


Matt Troutman 
m_troutman@hotmail.com


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